24-carat planet Orbiting a star that is visible to the naked eye, astronomers have discovered a planet twice the size of our own made largely out of diamond.

The rocky planet, called '55 Cancri e', orbits a sun-like star in the constellation of Cancer and is moving so fast that a year there lasts a mere 18 hours.

Discovered by a US-Franco research team, its radius is twice that of Earth's with a mass eight times greater. That would give it the same density as Earth, although previously observed diamond planets are reckoned to be a lot more dense. It is also incredibly hot, with temperatures on its surface reaching 1648°C.

"The surface of this planet is likely covered in graphite and diamond rather than water and granite," says Nikku Madhusudhan, the Yale researcher whose findings are due to be published in Astrophysical Journal Letters.

Diamond planets have been spotted before but this is the first time one has been seen orbiting a Sun-like star and studied in such detail.

"This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth," says Madhusudhan, adding that the discovery of the carbon-rich planet meant distant rocky planets could no longer be assumed to have chemical constituents, interiors, atmospheres, or biologies similar to Earth.

David Spergel, an astronomer at Princeton University, says it was relatively simple to work out the basic structure and history of a star once you know its mass and age.

"Planets are much more complex. This 'diamond-rich super-Earth' is likely just one example of the rich sets of discoveries that await us as we begin to explore planets around nearby stars."

"Nearby" is a relative concept in astronomy. Any fortune-hunter will need to travel about 40 light years, or 378 trillion kilometres, to reach it.